Norwalk virus is one of a group of small, noncultivatable, 27nm viruses which are important etiologic agents of epidemic gastroenteritis in older children and adults. The adaptation of biotinavidin immunoassays for detection of Norwalk virus antigen and for detection of seroresponses to the virus allowed us to perform an extensive survey of gastroenteritis outbreaks in Maryland nursing homes for the elderly. Serologic studies indicated that Norwalk virus was associated with gastroenteritis episodes in approximately one-fifth of the patients; this virus was the single most important agent associated with these outbreaks. Attempts to cultivate Norwalk virus are in progress employing a tissue culture system which has bee used successfully for cultivation of a porcine calicivirus. In addition, a porcine calicivirus has recently been obtained which will be used as a model for cloning of the Norwalk virus genome.